In Final Fantasy Tactics Advance, you can get a Target Bow in missions that offer Type 5 rewards. The Target Bow is selected randomly, so you'll have to walk around and return to a pub and look at the rewards for the missions until you spot a Target Bow.
These are the Type 5 items that will show up randomly in missions.
Vitanova, SaveTheQueen, Vigilante, El Cid Sword, Zorlin Shape, Colichemarde, Petalchaser, Spring Staff, Thor Rod, Life Crosier, Target Bow, Hades Bow, Aona Flute, Godhand, Giot Gun, Thief Hat, Mirror Mail, Ninja Gear, Black Robe, Feather Boots
In Final Fantasy Tactics A2, you get the Target Bow through the Bazaar. It is part of the Ornamented Bows B Set, and you will need: Great Serpent's Fang + Suspect Mushroom + Kuraisle Boxwood.